Video streams like television broadcasting are generally transmitted in the form of interlace scanning (hereinafter, “interlacing”) image data. In order to allow matrix-type display devices, such as liquid crystal display devices, or plasma displays, to display images by television broadcasting, pieces of image data of that video stream are converted into progressive scanning (hereinafter, “progressive”) image data.
Examples of a technique of converting interlacing image data into progressive image data are an inter-field scanning line interpolation process and an in-field scanning line interpolation process. An interpolation process inserts interpolated scanning line data in a space between individual scanning lines of interlacing image data in a video stream, thereby generating progressive image data corresponding to that video stream. The inter-field scanning line interpolation process and the in-field scanning line interpolation process are both interpolation processes in common, but have a following difference.
The inter-field scanning line interpolation process uses, as interpolated scanning line data, scanning line data of previous and next fields of interlacing image data in a video stream. In contrast, the in-field scanning line interpolation process uses, as interpolated scanning line data, scanning line data in the same field (more specifically, scanning line data right above and right below) of interlacing image data in a video stream.
The inter-field scanning line interpolation process and the in-field scanning line interpolation process can be applied simultaneously. For example, a signal processing device which changes the weighting of the inter-field scanning line interpolation process and that of the in-field scanning line interpolation process depending on the motion amount (change) of an image in a video stream is used. For example, Unexamined Japanese Patent Application KOKAI Publication No. H02-196581 discloses a signal processing device which selects the in-field scanning line interpolation process if the scanning line structure of image data in a video stream does not conform to NTSC (National Television System Committee).
Nowadays, editing private images using a video editing software through personal computers (hereinafter, “PC”), cell phones, and the like becomes possible. PCs and cell phones have various numbers of pixels of display screen and sizes thereof, so that the number of scanning lines also varies. Accordingly, in image editing, in addition to a conversion process into the foregoing progressive image, a scaling process of the progressive image (i.e., increasing/decreasing process of the scanning lines) are performed.
More specifically, the conversion process converts interlacing image data which is an original image into progressive image data by performing an inter-field interpolation process. The scaling process inserts scanning line data in the field at an appropriate interval (e.g., one line for each three lines) corresponding to the enlargement ratio of a screen (e.g., 360 lines to 480 lines) to the converted progressive image data.
Eventually, as odd-number lines are extracted in odd-number fields and even-number lines are extracted in even-number fields, the enlarged progressive image data is converted into interlacing image again. As shown in FIG. 21, the interlacing image data has scanning line data of an odd-number field (field 1) and scanning line data of an even-number field (field 2) mixed in the interlacing image date four line by four line.
As shown in FIG. 22, an explanation will be given of an example case in which a motion image that a vertical bar scrolls from the left to the right on the display is edited as an original image. FIGS. 23A to 23C are enlarged views of interlacing image data corresponding to the original image shown in FIG. 22. When inter-field scanning line interpolation is performed on those pieces of image data and the image data is enlarged, as shown in FIGS. 24A to 24C, image data having scanning lines of previous and next fields mixed therein is generated.
An explanation will now be given of an example case in which a signal processing device which performs in-field scanning line interpolation on a motion image and which performs inter-field scanning line interpolation on a still image displays image data generated thus way. The signal processing device determines that the image data is a frame of a motion image, and performs in-field scanning line interpolation. Accordingly, as shown in FIGS. 25A to 25C, image data having a position of the vertical bar shifted for each four lines is generated. In a display screen, not an image that one vertical bar scrolls from the left to the right (see, FIG. 22), but as shown in FIG. 26, an image that a vertical line waviness (having predetermined distortions) scrolls from the left to the right is displayed.
As explained above, when an image is edited using a video editing software on PCs or cell phones, a defect with interlacing may occur. Image data having a defective interlacing may cause unignorable display failure. Regarding a display failure, even a signal processing device disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H02-196581 cannot cope with it.